Coin discriminator where frequencies of eddy currents are measured

ABSTRACT

A coin processing machine with a coin discriminator operated by a method is provided. The coin discriminator measures how a coin having an iron core covered by a layer of copper, brass, or bronze affects a coil when the coin is subjected to magnetic fields generated by the coil, external to the coin. Eddy currents induced in the coin are detected external of the coin. The discriminator induces a magnetic field in the coil by driving the coil with time varying drive signals having high frequencies. The coin discriminator receives the coin at precise positions in the magnetic field and detects the eddy currents induced in the coin by measuring the eddy currents through the coil. Then, the coin discriminator compares the measured eddy currents with predetermined values for different types of coils, and determines the structure, materials and type of the coin.

TECHNICAL FIELD

The present invention relates to a method of identifying a metal coin.The method is used in a coin discriminator measuring how a metal coin,which has an metal core covered by a layer of another metal, affectscoil means when the coin reaches magnetic fields generated by the coilmeans external to the coin. Furthermore, the eddy currents induced inthe metal coin are detected by detection means external of the coin.

The present invention also relates to a coin processing machineincluding a coin discriminator as above type.

DESCRIPTION OF THE PRIOR ART

Coin discriminators are used for measuring different physicalcharacteristics of a coin in order to determine its type, e.g. itsdenomination, currency or authenticity. Various dimensional, electricand magnetic characteristics are measured for this purpose, such as thediameter and thickness of the coin, its electric conductivity, itsmagnetic permeability, and its surface and/or edge pattern, e.g. itsedge knurling. Coin discriminators are commonly used in coin handlingmachines, such as coin counting machines, coin sorting machines, vendingmachines, gaming machines, etc. Examples of previously known coinhandling machines are for instance disclosed in WO97/07485 andWO87/07742.

Moreover, methods and devices that measure the resistance orconductivity of a coin by exposing it to a magnetic pulse and detectingthe decay of eddy currents induced in the coin are generally known inthe technical field.

The way in which such coin discriminators operate is described in e.g.GB-A-2 135 095, in which a coin testing arrangement comprises atransmitter coil, which is pulsed with a rectangular voltage pulse so asto generate a magnetic pulse, which is induced in a passing coin. Theeddy currents thus generated in the coin give rise to a magnetic field,which is monitored or detected by a receiver coil. The receiver coil maybe a separate coil or may alternatively be constituted by thetransmitter coil having two operating modes. By monitoring the decay ofthe eddy currents induced in the coin, a value representative of thecoin conductivity may be obtained, since the rate of decay is a functionthereof.

Coin discriminators in prior art often employ a small coil with adiameter smaller than the diameter of the coin. The coil induces anddetects eddy currents in an arbitrary point of the coin, i.e. the actualpart of the coin, which is subject to the conductivity measurementabove, the eddy currents will vary depending on the orientation, speed,angle, etc., of the coin relative to the coil. This approach issufficient for a normal homogeneous coin made of a single metal or metalalloy.

However, in recent years new non-homogeneous coins have been issued indifferent countries. For example, these coins may contain bothbimetallic coins and iron coins covered in copper.

These new coins are very similar to some existing coins, i.e. they havealmost the same physical size and are made from the same or similarmaterials.

An iron core or disc forming an iron coin may be plated or clad with oneor more layers of copper or brass around either its whole surface oronly at both sides leaving its rim freely exposed as an iron rim.

All of the above-mentioned features make it difficult to discriminatebetween coins, especially between two iron coins having the samediameter of which one iron coin has a freely exposed iron rim and theother iron coin has an iron rim that is only partly or completely platedwith only a thin layer of copper, brass, or bronze.

One problem occurs when introducing new coins in different countries.This introduction means that coin accepting and counting machines mustdistinguish between the new coins and the existing national currencies.In most cases, this is not a problem. However, different coins withessentially the same dimensions may have the same “appearence” whenmeasured due to different manufacturing methods of the coins. Forexample, a type A of a coin is very similar to another coin, a type Bcoin. The type B and the type A coin are both iron coins. Thedifferences between these iron coins are the following. The type B ironcoins are clad in brass in comparison to the type A iron coins, whichare either plated or clad with copper. Another difference is that thetype B iron coins have the iron exposed on the rim and the type A ironcoins have a thin layer of copper over the rim. In theory, there is anaverage diameter difference of between these two types of A and B ironcoins. However, a small sample of the type A iron coins, which diameterswere measured with digital callipers, had diameters outside the theirspecified tolerances. The type B iron coins that have been in use a longtime tend to become smaller, especially the diameter. We expect to findtype B and type A iron coins with the same size.

Similarly, a type C and a type D coins are also difficult todiscriminate. Both coins are iron coins covered with copper. The type Ciron coin has a copper covered rim. The type D iron coin may have a thinsmear of copper on one side of the rim. This is due to the manufacturingmethod of this type D iron coin. This copper smear is created when thedie cutter punches out the coin, whereby a thin layer of copper may besmeared over a part of the edge, i.e. the rim, of the iron coin in thepunching direction.

The coin discriminators of the prior art described above fail to providea sufficiently accurate determination of the type of the above-mentionediron coins due to a similar effect on resistance for the coils measuringthe iron coins conductivity when the measured iron coins passes thecoils.

The coin measurement results obtained vary largely depending on theactual spot of measurement on the coin. If a given coin is measured at aposition located in the vicinity of the rim of a coin, which has a thincopper layer around an iron core, a coin with an iron core having anexposed, non-covered, iron rim may be mistakenly “seen” or discriminatedas being a coin with an iron core surrounded by a relatively thin copperor brass layer at all sides, i.e. over both the faces and the rim of theiron coin. Furthermore, the prior art solutions have problems inidentifying if the layers covering the rims of the iron coins are madeof copper, brass or bronze.

Moreover, iron coins with only a thin smear of copper, brass or bronzepartly covering the rim may be difficult to discriminate because theycan be “seen” as iron coins with both a non-covered rim or a coveredrim.

SUMMARY OF THE INVENTION

The main objects of the present invention are to allow repeatable andaccurate determination of coin types, i.e. coins comprising for examplean iron core covered completely or partly by a thin layer made ofanother metal such as copper, brass or bronze and having almost the samephysical size, and, in some cases, exactly the same size, by detectingresistance and inductance changes in the coil that measures the coin fordetermining if the coin has a covered or non-covered rim, and fordetermining the surface conductivity for the coin.

These objects are achieved by providing a coin processing machine with acoin discriminator operated by a method according to the invention. Themethod measures how a coin, which has for example an iron core coveredby a layer of another metal such as copper, brass, or bronze, affectscoil means when the coin is subjected to magnetic fields generated bythe coil means external to the coin. Eddy currents induced in the coinare detected by detection means external of the coin. The coindiscriminator induces a magnetic field in the coil means by driving thecoil means with time varying drive signals having high frequencies. Thecoin discriminator receives the iron coin at precise positions in themagnetic field. Then, the coin discriminator detects the eddy currentsinduced by the magnetic field in the iron coin by measuring the eddycurrents through the coil means, and compares the measured eddy currentsthrough the coil means with predetermined values for different types ofiron coins. Finally, the coin discriminator determines the structure,materials and type of the measured iron coin using the predeterminedvalues.

By providing a coin processing machine with a coin discriminatoroperated by a method according to the invention, the followingadvantages are obtained. The same coil means is used to make twomeasurements of the iron coin at different points of time, therebyeliminating the need and cost for an additional coil means and theadditional electronics that would have to be operatively connected tothe extra coil means. Other advantages are that the construction andmaintenance of the coin processing machine are simplified and theassociated costs for these measurements are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, reference being madeto the accompanying drawings, in which:

FIG. 1 is a schematic sectional view of an iron coin and a coindiscriminator according to the invention,

FIG. 2 is a schematic plan view of the relative positions between aniron coin in two different positions and the coin discriminator duringdiscrimination,

FIG. 3 is a block diagram of the electronic circuit used in the coindiscriminator in FIGS. 1 and 2,

FIG. 4 is a diagram over readings of frequency changes when threedifferent iron coins pass the coin discriminator,

FIG. 5 is a diagram over readings of resistances in the coindiscriminator when the three iron coins in FIG. 4 pass it,

FIG. 6 is a diagram over the three iron coins in FIGS. 4 and 5 withtheir iron rim readings highlighted, and

FIG. 7 is a block diagram of a coin processing machine comprising thecoin discriminator in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a coin discriminator 10 comprising a coil 20 mounted in ahousing 30. The coil 20 is connected to an electrical device (not shown)for supplying current pulses thereto. Voltage pulses may be used insteadof current pulses, this is a common knowledge for a skilled person. Acoin 40 is shown just as it reaches the generated magnetic field orpulses of the coin discriminator. Furthermore, the coin discriminatorcomprises detection means (not shown) for detecting changes inresistance and inductance of the coil 20 caused by the iron coinaffecting the magnetic pulses generated by the coil means in response tothe current pulses supplied from the electrical device.

In this embodiment, the coin is an iron coin 40 comprising a largeelectrical conductive core 50 of a first metal or alloy, e.g. iron orsteel, in the form of a disc. The coin core 50 is shown with dottedlines inside the coin 40 to the right in FIG. 2. The iron core 50 is alittle smaller than in reality so that the difference in size betweenthe outer periphery of the iron core and the outer contour of the coin40 is shown more clearly, i.e. exaggerated. This area between the outerperiphery of the iron core 50 and the outer contour of the coin 40 is athin layer of copper, brass, or bronze or any other metal used as theouter surface on coins, as is readily envisaged by a skilled person.

The coin discriminator 10 according to the invention makes twomeasurements, each measurement is done at different parts of each coin,i.e. at the coin core 50 and a coin rim 60, respectively. This will beexplained more in detail below. The detection means (not shown) of thecoin discriminator 10 determines the surface conductivity of the coin 40in one measurement by inducing eddy currents by means of the coil 20 inthe surface of the coin core 50. The other measurement determineswhether the coin has a freely exposed iron core 50 at the rim 60 or ifthe iron core, i.e. the rim, is covered with a thin layer 70 of anothermetal, e.g. copper, brass or bronze. A bond between the disc shaped coreand the layer is labeled 80. This bond 80 does not exist if the ironcore is not covered at the rim 60, as is understood by a skilled person.

The coil 20 in FIGS. 1 and 2 acts as a transmitter coil for exposing theiron coin 40 to a magnetic field. The exposure of the iron coin is donewhen it moves past the coin discriminator 10 along a coin rail 90 (shownin FIG. 2). The direction of movement for the iron coin is illustratedby a horizontal arrow pointing to the left in FIG. 1. Alternatively, theiron coin may move in the other direction, i.e. to the right, in FIG. 1,as is envisaged by a skilled person.

Furthermore, the coil 20 of the coin discriminator 10 shown in FIGS. 1and 2 also acts as a receiver coil being operatively connected tosuitable electronics, this will be explained in more detail later on inthis description, for detecting both the magnetic field variations, andmore particularly the inductance and resistance variations in the coil20 when a measured iron coin 40 passes it, and the surface conductivityof the measured iron coin and converting them into correspondingsignals. The signals are supplied to a detector (not shown), which isarranged to measure the decay and change of the signals and in responsedetermine a respective value of the inductance and resistance changes inthe coil 20 and the surface conductivity of the coin 40. The determinedsurface conductivity values for each measured iron coin 40 and theeffect of the iron coin on the inductance and resistance in the coil 20are subsequently used for identifying the type of the iron coin.

FIG. 2 shows the relative positions between one of the essential partsof the coin discriminator 10, i.e. the coil 20 in its housing 30, andthe iron coin 40 during the discrimination. In this embodiment, the coilis mounted about 3 mm above the coin rail 90, i.e. the lower end of thecoil is 3 mm above the rail. This position for the coil 20 depends onwhich type of iron coins that are going to be measured and the dimensionof the iron coins. Smaller or larger iron coins 40 may require othercoil positions in order to get accurate readings.

Each iron coin 40 moves past the coin discriminator 10 on the coin rail90 during the measurements. The discrimination according to theinvention is done at different points of time because the same coil 20is used for two measurements. One measurement determines if the ironcore, i.e. the rim 60 of the iron coin 40 is exposed or covered by athin layer of copper, brass, or bronze. The other measurement determinesif the iron coin core 50 is covered by a copper, brass, or bronze layer70. The copper, brass, or bronze layer on the iron coin core is detectedby measuring the conductivity of the surface for the iron coin 40. Thebare or covered iron coin/core rim 60 is determined by its magneticproperties, i.e its effect on the resistance and inductance for the coil20 when the iron coin rim reaches or passes the coil. Depending on themagnetic properties of the rim 60, the resistance and inductance of thecoil 20 will be influenced to different extents.

When the iron coin 40 moves from left to right in FIG. 2, the coin rim60 first reaches the coil 20, therefore the rim measurement is donefirst in this embodiment. The direction of movement for the iron coin isillustrated by a horizontal arrow placed to the left in FIG. 2 andpointing to the right. The iron coin could of course move in the otherdirection if desired. When the coin rim 60 is measured the iron coinshould leave about 75% of the coil 20 exposed, i.e. the coin rim coversabout 25% of the coil. Then, shortly afterwards, the centre of the ironcoin, more specifically, the iron coin core 50, reaches/covers the coiland the second measurement takes place. The rim measurement could ofcourse be done after the surface conductivity measurement is done. Thisis because the coin rim 60 passes the coil 20 once more before the ironcoin 40 is finally transported past the coil, as is readily understoodby a skilled person.

In FIG. 2, the iron coin 40 should preferably be placed over the centreof the coil 20 when the surface conductivity measurement takes place, asin the position to the right for the iron coin. When the coin rim 60 ismeasured the position for the iron coin should be as shown in FIG. 1with the magnetic field striking the rim of the iron coin. Additionally,the position for the coil 20 must be decided in relation to thedimensions of the coins that are going to be measured. The position forthe coil is chosen so that the rim 60 of the iron coin 40 does notaffect this second measuring of the surface conductivity. The distancebetween the coin rim 60 and the coil 20 should be larger thanapproximately 1 mm for securing an accurate reading of the surfaceconductivity.

The coil 20 of the coin discriminator 10 is small compared to thediameter of the iron coins 40 to be measured. The coil may have adiameter between 5 to 10 mm. Preferably, the ferrite core should have adiameter between 5 to 10 mm, but, preferably, a diameter of 7.3 mm. Theferrite may be between 2 to 6 mm, preferably, 3.7 mm high or thick andbe filled with a wire having a diameter between 0.08 to 1 mm,preferably, 0.2-mm. The wire should, preferably, be made of copper. Inprinciple, any small coil could be used in the coin detector ordiscriminator 10, as is envisaged by a skilled person. The use of aferrite pot core to direct the magnetic field makes thedetector/discriminator more efficient.

In a typical coin counting machine (not shown), the position of the ironcoin 40 is known from other sensors (not shown), as is envisaged by askilled person. This information is used to make the two measurements ofthe coin at different times using the same coil 20.

The surface conductivity is measured when the coil 20 is covered by theiron core 50 of the iron coin 40, as shown in FIG. 2. This means thatthe iron coin has its center essentially aligned with the center of thecoil 20 or the coin core 50 at least covers the whole coil. The durationof the current pulses supplied by the electrical device to the coil maybe chosen in accordance with the actual application.

To make the coil 20 work as a part of the coin detector/discriminator 10an electronic circuit 100 shown in FIG. 3 put a time varying currentthrough the coil. Changes in the current produce changes in the magneticfield produced by the coil 20, whereby the changing magnetic fieldproduces an electric current in the iron coin 40. This current in theiron coin is called an eddy current. The changing eddy current in turnproduces a changing magnetic field, which is measured by the coil 20.

If the same coil 20 is used for both generating and sensing the eddycurrents, the effect of the iron coin 40 is to cause an apparent changein the inductance and resistance of the coil. The electronic circuit 100measures these changes and uses them to identify the type of the ironcoin.

The electronic circuits used to measure iron coins 40 with a single coil20 can be divided into two types:

-   -   1. Continuous wave (CW) techniques that drive the coil 20 with a        continuous sine or square wave.    -   2. Pulse induction (PI) techniques that use a step change in        current to produce an exponentially decaying eddy current within        the iron coin 40.

The electronic circuit 100 in FIG. 3 drives the coin discriminator 10 byusing the continuous wave (CW) technique, which drives the coil 20 witha continuous sine or square wave. The CW electronics can be divided intotwo types:

-   -   1. Frequency shift    -   2. Phase shift

The first method, the frequency shift is the simplest and cheapest. Withthis technique, the coil 20 forms part of the frequency determiningelements of an oscillator 110. A change in the inductance of the coilcauses a change in the oscillator frequency. This frequency shift isused to identify the iron coin 40. The limitation of this simple methodis that it does not measure the change in the resistance of the coil 20,and, thus, it only uses half of the available information.

The second method, the phase shift method drives the coil 20, usually ata fixed frequency, and then measures the amplitude and phase of the coilvoltage or current. By measuring both amplitude and phase, the change ininductance and resistance for the coil can be calculated.

To separate a copper covered iron coin 40 from a brass or bronze coverediron coin according to the invention, the coin discriminator 10according to the invention uses high frequency eddy currents. The skindepth effect will make these currents flow mainly in the copper, brassor bronze layer. The skin depth effect when using AC-power instead ofDC-power is a physical effect that is common knowledge for a skilledperson.

In this embodiment, a type A, a type B, and a type D iron coin are usedto explain the function of the coin discriminator 10 according to theinvention. These coins are quite similar and good examples of referenceiron coins 40. Alternatively, any other type of existing or future ironcoin with a large iron core 50, which is completely or partly covered bya thin layer of copper, brass or bronze may of course be used, as isenvisaged by skilled person.

The type A and the type B coin are made of iron clad in brass. A type Eand the type D coins are iron coins clad in copper. A type F, the type Cand the type A coins are iron coins either plated or clad in copper. Thetype E and D coins often have a copper smear on the rim 60, i.e. thecopper smear only partly covers the rim. The brass plating has aspecified thickness of 0.068 mm. The skin depth in 25% IACS brass willbe this distance at 3.7 MHz. This means that we must use a frequencyover 3.7 MHz to “hide” the iron core 50, i.e. a lower frequency wouldmake the eddy current penetrate further into the coin, thereby“revealing” or reaching the iron core.

The 25% IACS brass is defined according to the International AnnealedCopper Standard (IACS) scale. This scale relates to the conductivity ofmetals. On this scale, the conductivity of pure annealed copper is takenas 100%, the bronze used in “copper” coins is about 50%, and brass istypically 25%. The gold alloy in some coins is about 16% and thecopper-nickel alloy used in “silver” coins is just over 5%. This isreadily understood by a skilled person.

The maximum frequency used in the coin discriminator 10 is alsodetermined by the skin depth effect, which, in this embodiment, is theskin depth in the copper wire of the coil 20. Because the current onlyflows on the surface of the wire, the resistance is greater than itsresistance when DC power is used. From this point of view, a frequencyas low as possible is preferred.

Based on these Skin depth arguments, the preferred frequencies is in therange of 4 to 10 MHz but, preferably, between 5 to 8 MHz when used inthe coin discriminator 10 according to the invention.

It is possible to design electronics to accurately measure the change ininductance and resistance of a coil 20 at these frequencies using thephase shift method. However, the electronic circuit 100 in FIG. 3 willnot be simple or cheap because of the high frequencies used. The uniquefeature of the electronic circuit 100 in the coin discriminator 10according to the invention is the use of a frequency shift method thatcan accurately measure changes in both inductance and resistance fastand reliable.

In FIG. 3, an electronic circuit 100 for operating the coindiscriminator 10 is shown as a block diagram. A voltage controlledoscillator 110 runs at a frequency of eight times the self-resonancefrequency for the coil 20. A divide by 8 circuit 120 generatesfrequencies at the coil resonance with phases of plus and minus 45°. Acontroller 130 selects one of these two phases via a selector device 200and drives the coil 20 via a bi-directional current source 170. Thevoltage across the coil 20 will be a sine wave. The output from acomparitor 140 is a logic level square wave with the same zero crossingsas the sine wave in the coil 20. This square wave is compared with areference phase of 90° from the divide by 8 circuit 120. The comparisonis done via an exclusive OR gate 150 followed by a low pass filter 160to the left in FIG. 3. The low pass filter comprises two main componentsof which none is explained in more detail, such a low pass filter is acommon knowledge for a skilled person. The output voltage from the lowpass filter 160 is used to control the frequency of the oscillator 110.The constant current source 170 is used for driving the coil 20. A16-bit counter 180 measures the frequency of the voltage-controlledoscillator 110. A detector interface 190 is used for connecting thecontroller 130, i.e. the electronics driving the coin discriminator 10,to other electronics (not shown). The other electronics could be anykind of suitable hardware and software, e.g. a PC, used for furtherprocessing of the measurement results, e.g. presentation of the resultfor an operator of the coin counting and sorting machine, or a processorin a coin counting and sorting machine.

If the current source 170 were driven by a zero degrees phase, theelectronic circuit 100 would lock to the resonant frequency of the coil20. The electronic circuit would then be recognised as an example of aknown type, which is called a phased locked loop and common knowledgefor a skilled person.

By driving the current source 170 with a phase of 45°, the electroniccircuit 100 will still lock. However, the frequency will produce a phaseshift of 45° between the voltage and current through the coil 20. Fromthe physics it is known that this 45° phase shift only occurs at the “3dB points” on the resonance curve. By using phases of both plus andminus 45° the frequencies of the upper and lower “3 dB points” can bemeasured. The average of these two frequencies is the resonancefrequency for the coil 20. The difference between the frequencies is thewidth of the resonance frequency.

The 16 bit counter 180 measures the frequency of the voltage-controlledoscillator 110. The controller 130 does this by counting how many cyclesoccur in a fixed period. In this embodiment, a period may be in therange of 50 to 200 μs but a period of 125 μs is preferred, so that thecount gives the frequency change in kHz. The controller also interfacesto the rest of the electronics, i.e. the detector interface 190connected to other components (not shown) of a coin counting and sortingmachine 700 shown as a block diagram in FIG. 7.

In FIG. 4, readings from the preferred coin discriminator 10 accordingto the invention are shown. The y-axis on the graph shows the frequencychanges in kHz caused by the three iron coins 40 passing the coil 20.From left to right there are the type B, the type D and the type A coin.

In FIG. 4, the x-axis is simply distance in mm along the coin rail 90.The solid line is the change in the centre or resonance frequency of theoscillator 110. The coil 20 used in FIG. 4 is the coil shown in all ofthe earlier drawings. With this coil, the no-coin centre or resonancefrequency is between 4 to 10 MHz, preferably, 5 MHz. The first iron coin40, the brass plated type B, increases this frequency by 600 kHz. Thisfrequency change is range dependent. The above graph was made with acoil to coin range of 0.9 mm.

The effect from the iron coin 40 on the resistance of the coil 20depends on the range or distance between the iron coin and the coil. Theeffect from the iron coin on the resistance for the coil decreases asthe distance between the iron coin and the coil increases. The decreaseof the coil resistance occurs in proportion to the diameter of thecircularly flowing eddy current in the coin. The effect from the coin 40on the inductance of the coil 20 also depends on the range or distancebetween the iron coin and the coil. The effect on the inductance for thecoil decreases as the distance between the iron coin and the coilincreases. The decrease of the coil inductance occurs in proportion tothe diameter raised to a second power of the circularly flowing eddycurrent in the coin, i.e. in proportion to the area covered by thecircularly flowing eddy current.

The dotted line is the width of the resonance frequency, this is thefrequency difference between the upper and lower “3 dB points”. Thewidth of the resonance frequency is a direct measurement of resistancein the coil 20. The wider the resonance frequency, then the higher theresistance. The dotted line demonstrates this effect. The type B ironcoin 40 has a resonance frequency width of 570 kHz compared to 530 kHzfor the other two iron coins. This is because brass has a higherresistance than copper, i.e. copper is a better conductor than brass.The dotted line shows that without a coin 40, the resonance frequencywidth is 430 kHz. This is due to the resistance of the wire in the coil20.

The text book method for finding the resistance of a coil 20 is from its‘Q’ or quality factor. A high value of Q implies a low coil resistance.The Q and the self-resonant frequency of a coil are given by theequations:

The self-resonant frequency is $f_{0} = \frac{1}{2\pi\sqrt{LC}}$

The Q of a coil 20 is$Q = {\frac{2\pi\quad{fL}}{R} = \frac{f}{\Delta\quad f}}$Where:

-   -   L is the inductance of the coil 20    -   C is the total capacitance in parallel with the coil    -   R is the resistance of the coil at the resonance frequency    -   f₀ is the self resonant frequency    -   Δf is the frequency difference between the 3 dB points.

The readings from the three iron coins 40 in FIG. 4 have been used toproduce the graph in FIG. 5.

In FIG. 5, the curve shows the Q of the coil 20 dropping each time themeasured iron coin 40 passes over it. The graph shows the greater dropfor the high resistance brass plated type B iron coin to the leftcompared to the two copper plated iron coins, i.e. the type D in themiddle and the type A to the right.

The graph in FIG. 6 shows the same iron coin readings as in FIG. 4 and 5but, here, the readings are processed to highlight the readings of theiron rims on each iron coin 40. As before the type B followed by thetype D followed by the type A coin pass the coin discriminator 10 andtwo measurements for each iron coin are done.

FIG. 6 simply illustrates the resonance frequency width minus onequarter of the centre frequency shift. The processing must be simplebecause of the high speed of the iron coins 40 passing through the coindiscriminator 10 and the coin counting and/or sorting machine 700 shownin FIG. 7.

The iron rims on the brass plated type B iron coin 40 are shown moreclearly than on the copper plated type D iron coin. This is because oftwo effects pulling in opposite directions. The magnetic properties ofthe iron are trying to increase the inductance of the coil 20, whereasthe eddy currents in the surface of the iron coin are trying to reducethe coil inductance. The low resistance copper plated type D iron coinallows a greater eddy current and thus hides more of the iron at the rim60 of the iron coin 40.

The copper covered rims 60 of the type A iron coin 40 hide the ironcompletely at these high frequencies. This means that the inductance ofthe coil 20 only decreases as this type A iron coin passes over it.

Referring now to FIG. 7, the coin processing machine 700 according toone aspect of the present invention is schematically illustrated. In anexemplifying but not limiting sense, the coin processing machine 700 ofFIG. 7 is selected to be a coin sorter. The mass/es of coins 40, whichare to be sorted by the machine 700, are deposited into a coin inlet710. Here, the coins may be any types of coins not just iron coinscovered by a thin layer of copper, brass or bronze, as is readilyunderstood by a skilled person. The coins are fed by a coin feeder 720,such as a hopper and/or an endless belt, to the coin discriminator 10,which has been described above with reference to FIGS. 1, 2, and 3. Thecoin discriminator 10 is operatively connected to a logic device 732 inthe form of a CPU, which is operatively connected to a memory 734, suchas a RAM, ROM, EEPROM or flash memory. The memory 734 stores a set ofcoin reference data, which is used by the logic device 732 todiscriminate among the coins 40 received through the coin inlet 710.More specifically, the coin reference data relates to typical values ofconductivity and permeability for all different types of coins and totypical values of the effect from all different types of coins on theresistance and inductance of the coil 20, that the coin processingmachine 700 is capable of processing.

The logic device 732 is programmed to receive measurement data obtainedby the coil 20 and the controller 130, which is operatively connected tothe detector interface 190, for storing the data relating to the surfaceconductivity of the coin 40, and resistance and inductance changes ofthe coil 20 when the coin passes it. Once these measurement data havebeen received for a coin, the logic device 732 will read the coinreference data stored in the memory 734, which also is operativelyconnected to the detector interface 190, and search for any matches. Ifthe physical and magnetic properties for the iron coin measured by thecoin discriminator 10 correspond to one specific iron coin type definedby the iron coin reference data, then the type of iron coin has beenpositively identified. Otherwise, the iron coin 40 is of an unknown typeand handled by a coin reject device 740, which preferably will deliverthe iron coin through an external opening in the machine 700, so thatthe iron coin may be removed by a user. The rejected iron coin 40 mayalso be re-circulated back into the coin discriminator 10 for anotherattempt to discriminate it.

The coin types defined by the coin reference data in the memory 734 maypreferably relate to the denomination and currency of each differenttype of coin 40, which is to be handled by the coin processing machine700.

Once the type or identity of the coin 40 has been determined by the coindiscriminator 10 and the logic device 732, the coin is passed to a coinsorter 750, which uses the identified coin type to sort the coin 40 intoone specific coin box, etc., in a coin storage 760. The coin boxes,etc., in the coin storage are preferably externally accessible for theuser of the machine 700.

A future development of the coin discriminator 10 according to theinvention would be to use more than one coil 20 if the coins 40 to bemeasured would have a larger diameter or thickness than the coinsmeasured in this embodiment. In this case, the coils may have to beplaced in different positions in relation to each other to be able tocover coins with different diameters, e.g. higher or lower in relationto the coin rail 90 and the other coil, in order to make accuratemeasurements of each coin 40.

1. A method of operating a coin discriminator by measuring how a metalcoin affects coil means when the coin is subjected to magnetic fieldsgenerated by the coil means external to the coin, and wherein eddycurrents induced in the coin are detected by detection means external tothe coin, the method characterized by: inducing a magnetic field in thecoil means forming part of a resonant circuit of the coin discriminatorby driving the coil means by time varying signals at frequencies near anatural resonance of the coil means, measuring frequencies of eddycurrents, induced by the magnetic field in the coin, near the resonance,through the coil means at a first and second position of the coin, andcomparing the measured frequencies with predetermined values fordifferent types of coins for determining a type of the coin.
 2. A methodof operating a coin discriminator according to claim 1, characterized inthat said frequencies are measured preferably at the 3 dB points.
 3. Amethod of operating a coin discriminator according to claim 1,characterized in that the step of comparing the measured frequenciescomprises the step of: determining a shift in width and resonantfrequency of the coil means from the measured frequencies, and comparingthe determined width of the resonant frequency with predetermined valuesfor different types of coins for determining the type of the coin.
 4. Amethod of operating a coin discriminator according to claim 3,characterized in that the step of comparing the determined width of theresonant frequencies with predetermined values comprises the steps of:converting the frequencies into changes in inductance and resistance forthe coil means, comparing the changes in inductance and resistance forthe coil means with predetermined values for different types of coins.5. A method of operating a coin discriminator according to claim 1,characterized in that: the eddy currents through the coil means aremeasured at an edge portion of the coin being positioned in the magneticfield generated by the coil means, wherein it is determined whether themeasured edge portion of the coin is covered by a layer of anothermetal.
 6. A method of operating a coin discriminator according to claim1, characterized in that: the eddy currents are induced in a surface ofthe center portion of the coin and are detected by measuring the eddycurrents through the coil means.
 7. A method of operating a coindiscriminator according to claim 6, characterized by the steps of:converting the measured eddy currents through the coil means into valuesof conductivity for the coin, comparing the values of conductivity forthe coin with predetermined values for different types of coins, anddetermining the type of the measured coin by using the predeterminedvalues.
 8. A method of operating a coin discriminator according to claim1, wherein the method drives the coil means with time varying drivesignals having a frequency between 4 to 10 MHz, preferably, between 5 to8 MHz, for surface conductivity measurement or lower frequencies tomeasure the bulk conductivity of the coin.
 9. A method of operating acoin discriminator according to claim 1, wherein said metal coin is aniron coin.
 10. A method of operating a coin discriminator according toclaim 1, wherein said metal coin has a metal core covered by a layer ofanother metal, preferably copper, brass, or bronze.
 11. A coindiscriminator configured to measure how a metal coin affects coil meansof the discriminator when the coin is subjected to magnetic fieldsgenerated by the coil means external to the coin, and wherein eddycurrents induced in the coin are detected by detection means of thediscriminator external to the coin, the method characterized by: anoscillator circuit adapted to induce a magnetic field in the coil meansby driving the coil means at frequencies near a natural resonance of thecoil means, an eddy current detector adapted to detect and measurefrequencies of eddy currents, induced by the magnetic field in the coin,through the coil means at a first and second position of the coin, meansfor comparing the frequencies of the measured eddy currents through thecoil means with predetermined values for different types of coins fordetermining a type of the coin.
 12. A coin discriminator according toclaim 11, characterized in that said frequencies are measured preferablyat the 3 dB points.
 13. A coin discriminator according to claim 11,characterized by: a converter adapted to convert the frequencies intochanges in inductance and resistance for the coil means, wherein saidmeans for comparing is adapted to compare changes in inductance andresistance for the coil means with predetermined values for differenttypes of coins.
 14. A coin discriminator according to claim 11,characterized in that said discriminator and the eddy current detectoris adapted to detect the eddy currents at an edge portion of the coin.15. A coin discriminator according to claim 14, characterized in thatsaid discriminator is adapted to determine if the edge portion of thecoin is covered by a layer of another metal.
 16. A coin discriminatoraccording to claim 11, wherein said discriminator and the eddy currentdetector are adapted to detect the eddy currents at a center portion ofthe coin.
 17. A coin discriminator according to claim 16, characterizedby: a converter adapted to convert parameters of the measured eddycurrents through the coil means into values of surface conductivity forthe coin, and means for comparing the values of surface conductivity forthe coin with predetermined values for different types of coins.
 18. Acoin discriminator according to claim 11, wherein the discriminator isadapted to drive the coil means with time varying drive signals having afrequency between 4 to 10 MHz, preferably, between 5 to 8 MHz.
 19. Acoin discriminator according to claim 11, wherein the eddy currentdetector is adapted to measure two or more frequencies near theresonance, preferably the 3 dB points, for determining the shift inwidth and resonant frequency of the coil means.
 20. A coin discriminatoraccording to claim 19, wherein said means for comparing is adapted tocompare the determined width of the resonant frequency of the coil meanswith predetermined values for different types of coins.
 21. A coindiscriminator according to claim 11, wherein said metal coin is an ironcoin.
 22. A coin discriminator according to claim 11, wherein said coinhas a metal core covered by a layer of another metal, said another metalis copper, brass, or bronze.
 23. A coin processing machine comprising acoin inlet, a coin feeder, and a coin processor, characterized in thatthe coin processing machine comprises a coin discriminator according toclaim
 11. 24. A coin processing machine according to claim 23, whereinthe coin discriminator is coupled to the coin processor and is adaptedto determine a type, identity or denomination of respective coinsreceived from the coin feeder, and is adapted to supply the determinedtype, identity or denomination to the coin processor, characterized inthat the coin processing machine comprises: the coin discriminatorpositioned to induce and detect eddy currents in a center portion and anedge portion, respectively, of the coin; a storage device operativelyconnected to the coin discriminator and adapted to store coin referencedata; and a logic device operatively connected to the coin discriminatorand adapted to determine an identity of the coin by comparing said coinreference data to data obtained from the detected eddy currents andrelated to an effect of the coin on magnetic properties of the coindiscriminator and a surface conductivity of the coin.